1. Field of the Invention
The invention relates to methods for the fabrication of mask type color television tubes and, more particularly, to optical systems used to illuminate the screens of these tubes during their manufacture. The invention also concerns a diaphragm for these optical systems.
2. Description of the Prior Art
A color television tube has (FIG. 1) a front envelope 1, on the internal face of which is deposited the screen. This screen is usually formed by vertical stripes such as those referenced 2, made of cathodoluminescent materials (luminophors) which, when excited by an electron beam produced by an electron gun, emit a red, green or blue light. The screen thus has a sequence of sets of three vertical stripes, each set having a red stripe, a green stripe and a blue stripe, and each stripe is excited by a corresponding electron beam. To make the electron beam intended for one color stripe, for example for the blue stripe, strike only the lumiphor which has to produce this color, there is provision for placing a perforated mask 3 before the screen. The position and arrangement of the openings (for example the elongated slits (4) of this perforated mask 3, in the vertical direction, ensure the selection of the colors in combination with the corresponding electron beam.
Since the position of the mask with reference to the screen has to be determined with precision, the mask 3 is used to make the screen. To this effect, it is fixed to the envelope 1 of the tube before the screen is formed. Each luminophor is then placed as follows: the internal face of the envelope is coated with a solution, containing the luminophor to be deposited and a photosensitive material which hardens when illuminated by ultra-violet (UV) radiation, or by a mixture of UV radiation and blue light. This solution is then illuminated by means of an optical system 5 which includes an UV radiation source 6 and a lens 7 with a diaphragm 8. The optical system 5 simulates the deflection of the electron beams of the tube. The position of the optical system 5, notably that of the UV lamp 6, depends on the color which it is sought to obtain on the screen so as to create the sequence of red, blue and green vertical stripes. Of course, to each position of the optical system, there corresponds a earlier deposition of a determined solution of luminophor and photosensitive material.
To obtain these differently colored vertical stripes, the optical system is shifted horizontally. Since the screen is illuminated by means a mask 3, perforated with elongated slits 4 in the vertical direction, the result thereof is that each vertical stripe 2 is not illuminated uniformly throughout its height and, therefore, does not have a constant width. In particular, narrowings or contractions 9 appear at those places of the screen which correspond to the spaces 10 between the slits 4 of the mask. Furthermore, this effect varies according to the position on the screen, namely the inclination of the UV rays with respect to the perpendicular direction to the center of the screen.
It is desirable for the vertical stripes to be as uniform as possible throughout the surface of the screen. To obtain this uniformity, there is provision, during each illumination step after the deposition of a luminophor, for shifting the optical system 5, including the UV source 6, vertically with reference to the screen or the UV source, or again for applying this shift to the diaphragm 8 alone.
It is desirable for the vertical stripes to be as uniform as possible throughout the surface of the screen. To obtain this uniformity, there is provision, during each illumination step following a deposition of a luminophor, for modifying the illumination of the screen so as to shift the projected image of the mask vertically. This can be obtained in different ways, such as the shifting of the optical system 5 with respect to the envelope/mask set, or conversely.
For reasons of convenience and ease of implementation, it is preferable to shift the optical system 5 vertically with respect to the envelope/mask set. In an illumination operation of this type, comprising a vertical movement of the optical system, it is important that there should be no local heterogeneity of light energy due to the shadows caused by the opaque parts between the slits 4 of the mask 3, and this leads to the use of a diaphragm 8, the aperture of which has a defined shape. Thus, the vertical height V of the aperture varies as a function of the x and y coordinates of the point to be eliminated.
A vertical shift of this type, with a determined value associated with a determined diaphragm aperture, gives good results but it is not possible to obtain optimum definition and uniformity of the luminophor stripes.
An aim of the present invention, therefore, is the implementation of a method for the illumination of the different luminophor stripes that makes it possible to obtain definition and uniformity of the luminophor stripes as close as possible to the optimum values.
Another aim of the present invention is also the making of a diaphragm which can be used to implement said illumination process.
Another aim of the present invention is to determine the aperture of the diaphragm.